Giant Transition-State Quasiparticle Spin-Hall Effect in an Exchange-Spin-Split Superconductor Detected by Nonlocal Magnon Spin Transport

Abstract

Although recent experiments and theories have shown a variety of exotic transport properties of nonequilibrium quasiparticles (QPs) in superconductor (SC)-based devices with either Zeeman or exchange spin-splitting, how a QP interplays with magnon spin currents remains elusive. Here, using nonlocal magnon spin-transport devices where a singlet SC (Nb) on top of a ferrimagnetic insulator (Y3Fe5O12) serves as a magnon spin detector, we demonstrate that the conversion efficiency of magnon spin to QP charge via inverse spin-Hall effect (iSHE) in such an exchange-spin-split SC can be greatly enhanced by up to 3 orders of magnitude compared with that in the normal state, particularly when its interface superconducting gap matches the magnon spin accumulation. Through systematic measurements by varying the current density and SC thickness, we identify that superconducting coherence peaks and exchange spin-splitting of the QP density-of-states, yielding a larger spin excitation while retaining a modest QP charge-imbalance relaxation, are responsible for the giant QP iSHE. The latter exchange-field-modified QP relaxation is experimentally proved by spatially resolved measurements with varying the separation of electrical contacts on the spin-split Nb.

Giant enhancement to spin battery effect in superconductor/ferromagnetic insulator systems

“…Here we consider the spin battery effect [5] that is the static spin accumulation of Bogoliubov quasiparticles in a superconductor (SC) generated either by the coherent FMR drive or by the thermal magnons in the adjacent FI. Our study is motivated by the recent experiment demonstrating that magnons induce a giant inverse spin-Hall signal in the transition state of the Nb/YIG superconductor/ferromagnetic insulator system [34]. Due to the close relation between the spin Hall signal and spin density, this observation hints that the spin accumulation induced by thermal magnons is modified in a highly nontrivial way by the superconducting correlations.…”

“…We expect this effect also to explain the giant spin-Hall signal measured in Ref. [34], but its precise description would require appending the theory with the description of the spin-Hall angle [75][76][77].…”

“…Most of the experimental [13,15,16,[29][30][31][32][33][34] and theoretical works studying magnetization dynamics in superconductor/ferromagnet systems focus on the FMR properties [35][36][37][38][39][40][41] and spin torques [42,43]. Here we consider the spin battery effect [5] that is the static spin accumulation of Bogoliubov quasiparticles in a superconductor (SC) generated either by the coherent FMR drive or by the thermal magnons in the adjacent FI.…”

“…Next, we consider the spin accumulation driven by the stochastic magnetization corresponding to the magnon thermal field at temperature T m = T which can be controlled with the help of electrical spin injection based on the spin Hall effect [11,34]. For that we find μ sz by averaging Eq.…”

“…For YIG [64], m M ≈ 1 eV −1 Å −2 and [65] v s ≈ 500 Å 3 . Figure 4 shows S(T ) for parameters qualitatively corresponding to the EuS/Al and YIG/Nb based FI/SC bilayers that have been studied recently [34,49]. The spin signals are enhanced even more due to the energy dependence of the inelastic scattering rate when the relaxation is due to the electron-phonon coupling.…”